In conventional high volume photofinishing, a photoprocessor receives exposed undeveloped film. This film is chemically developed at the photoprocessor and the developed images are then printed optically at a printing station from onto a photosensitive paper web, one complete frame at a time. The web is transported in a lengthwise direction from an input cassette, past the printing station and into an output cassette, pausing at the print station for a sufficient time to allow exposure of one image frame after another from the negative onto sequential locations on the web. The drive mechanism for the web has previously been designed to cause it to form a slack loop immediately before and after the print station. These slack loops act as buffers, allowing the drive mechanism to continuously withdraw the web from the input cassette and feed it into the output cassette without interruption despite the pausing of the web at the print station. Incorrect positioning of the web at the print station is usually not critical since some small space is allowed for in the lengthwise direction of the web, between printed images for later cutting. Thus, any slight error in positioning of the web at the print station typically only causes the size of this space to vary somewhat. The size of each slack loop has been monitored by a dual emitter/detector system. In such a system an optical or acoustic emitter is positioned to direct a beam perpendicular to the direction in which the slack loop extends, with a corresponding optical or acoustical detector being positioned at the other side of the loop to receive the beam. Two such emitter/detector sets are provided, each spaced from the other in the direction in which the slack loop extends. Such a configuration is illustrated, for example, by loop detector 6 in U.S. Pat. No. 4,878,067 which uses a light emitter (although acoustic emitters have also been previously used in place of the light emitters).
It has been recently suggested that photofinishers adopt a digital environment in which developed images on the films are scanned to yield corresponding digital images, or digitally captured images are received from digital cameras or remote scanners. These digital images are then subjected to any desired digital image processing, and the resulting digital images are printed by a digital printer, such as a laser printer. Laser printers use a rotating platen over which a photographic paper web can pass, with the laser printing by scanning one line at a time in a direction across the web. Continuous movement of the paper at a precise velocity in the lengthwise direction of the web, provides for scanning in the other direction (that is, in the lengthwise direction of the web). A simple laser printer configuration with slack loops, is also illustrated in U.S. Pat. No. 4,878,067. However, the present invention recognizes that the size of the slack loops is not known with much precision, since the loop detector can only tell that the most extreme portion of the slack loop (the loop "meniscus") is somewhere between the two beams. The present invention further recognizes that in the case of a laser printer or other line by line printer, the need for precise movement of the web is particularly critical. In the case where slack loops on either side of the print station vary in size, this leads to variable and unequal web weights which in turn can cause minor variations in the advancing of the web through the print station. Further, the present invention recognizes that in printing images in particular, the widths of the web might change. This may require the size of the slack loops to be adjusted. However, the present invention further realizes that with the type of slack loop detector system in U.S. Pat. No. 4,878,067, there is no easy way to reconfigure the printer for substantially different sized slack loops without physically moving the location of the entire slack loop detector system.
It would be desirable then, if a slack loop detector in a web transport of an image printer, could be provided which accurately tracks the size of the slack loop. Such accurate detection would be particularly important in a laser printer or other line by line printer where the web should be precisely advanced past the print station. Further, it would be desirable if a means can be provided which allows the slack loop detector to readily detect slack loops of various sizes without cumbersome repositioning of the detector or its components.